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| United States Patent |
5,519,172
|
|
Spencer
, et al.
|
May 21, 1996
|
Jacket material for protection of electrical conductors
Abstract
The present invention is an improved jacketing material for all forms of
electro-magnetic energy conductors. The jacket material of the present
invention comprises a silicone material that is imbibed into a porous
polymer, such as expanded polytetrafluoroethylene, to produce a flexible
and durable composite. When applied as a cable jacket, the composite
material of the present invention produces dramatic improvement over the
use of silicone alone, especially in the areas of load sharing, resistance
to fatigue from repeated flexure, and resistance to harsh environmental
conditions. The ability of the material of the present invention to
withstand repeated autoclave cycles without compromise makes it
particularly suitable for cable jacketing in bio-medical applications.
| Inventors:
|
Spencer; Mark S. (Phoenix, AZ);
Rubin; Edward (Phoenix, AZ)
|
| Assignee:
|
W. L. Gore & Associates, Inc. (Newark, DE)
|
| Appl. No.:
|
321634 |
| Filed:
|
October 11, 1994 |
| Current U.S. Class: |
174/110R; 174/36; 174/110F; 174/110S; 174/110FC; 174/120SR |
| Intern'l Class: |
H01B 007/28 |
| Field of Search: |
174/110 R,110 S,110 FC,110 F,110 PM,120 SR,120 AR,36
|
References Cited
U.S. Patent Documents
| 3150207 | Sep., 1964 | Gore | 260/827.
|
| 3217083 | Nov., 1965 | Gore | 174/25.
|
| 3278673 | Oct., 1966 | Gore | 174/120.
|
| 3953566 | Apr., 1976 | Gore | 264/288.
|
| 4187390 | Feb., 1980 | Gore | 174/102.
|
| 4279245 | Jul., 1981 | Takagi et al. | 128/4.
|
| 4347204 | Aug., 1982 | Takagi et al. | 264/127.
|
| 4557957 | Dec., 1985 | Manniso | 428/36.
|
| 4613544 | Sep., 1986 | Burleigh | 428/315.
|
| 4720400 | Jan., 1988 | Manniso | 427/243.
|
| 4791966 | Dec., 1988 | Eilentropp | 138/154.
|
| 4862730 | Sep., 1989 | Crosby | 73/38.
|
| 4978813 | Dec., 1990 | Clayton et al. | 174/117.
|
| 5237635 | Aug., 1993 | Lai | 385/101.
|
| 5286924 | Feb., 1994 | Loder et al. | 174/117.
|
| 5362553 | Nov., 1994 | Dillon et al. | 428/246.
|
| 5437900 | Aug., 1995 | Kuzowski | 428/36.
|
| Foreign Patent Documents |
| 0441140 | Aug., 1991 | EP.
| |
| 2696347 | Apr., 1994 | FR.
| |
| 94/07565 | Apr., 1994 | WO.
| |
Other References
"Silicone and Poly(tetrafluoroethylene) Interpenetrating Polymer Networks,"
by Mark E. Dillon, copyright 1994, American Chemical Society, pp. 394-404.
|
Primary Examiner: Nimmo; Morris H.
Attorney, Agent or Firm: Johns; David J.
Parent Case Text
The present application is a continuation-in-part of U.S. patent
application Ser. No. 08/305,477, filed Sept. 13, 1994, now abandoned.
Claims
The invention claimed is:
1. A jacketed conductor comprising
at least one conductor sealed within a jacket, the jacket comprising
a polymeric material including a microporous structure extending between a
top and a bottom surface; and
a silicone filler that fully envelops the microporous structure between the
top and bottom surfaces to seal the microporous structure and render it
liquid impermeable.
2. The jacketed conductor of claim I wherein the polymeric material
comprises a scaffold of polymeric nodes interconnected by fibrils, with
the nodes and fibrils enveloped with the silicone.
3. The jacketed conductor of claim 2 wherein the polymeric material
comprises expanded polytetrafluoroethylene (PTFE).
4. The jacketed conductor of claim 1 wherein the jacket is elastic in an
axial dimension.
5. The jacketed conductor of claim 4 wherein the jacket is capable of
withstanding compression in its axial dimension one dimension of 75
percent with a rebound of at least 50 percent to its original dimensions.
6. The jacketed conductor of claim 1 wherein the jacket is inelastic in a
longitudinal dimension so as to load share longitudinal strain between the
jacket and the conductor.
7. The jacketed conductor of claim 1 wherein the jacket is capable of
withstanding repeated exposure to temperatures in excess of 250 .degree.
C. without significant degradation.
8. The jacketed conductor of claim 7 wherein the jacket is capable of
withstanding repeated exposure to harsh chemicals without significant
degradation of the jacket and conductor.
9. The jacketed conductor of claim 1 wherein the silicone is selected from
the group consisting of methylhydrogen siloxane, dimethylhydrogen
siloxane, dimethyl siloxane, dimethylvinyl-terminated siloxane,
dimethylmethylphenylmethoxy silicone polymer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to protective jacket material compositions
with improved physical and electrical performance characteristics.
2. Description of Related Art
Electro-magnetic energy conductors, whether transmitting electrical current
or light signals, are conventionally coated with a protective jacket. Such
jackets may serve one or more of a host of functions, including providing
electrical and/or thermal insulation, serving as a sheath to assist in
containing multiple conductors, and providing physical protection for the
conductor from attack or damage from environmental conditions or stresses
applied to the conductor during use.
Among the successful jacket materials generally used today are polyvinyl
chloride (PVC), polyurethane, polyimide, polytetrafluoroethylene (PTFE),
expanded PTFE, fluorinated ethylene propylene (FEP), perfluoroalkoxy
polymer (PFA), polyesters, silicone rubber, and nylon. These materials may
be applied over the conductors in a variety of ways, including by
extrusion, tape wrap, insertion within pre-formed tubes, shrink wrap. etc.
The choice of material or materials and the manner or manners of
application over a conductor are all design choices heavily dependent upon
the required properties sought and the conditions anticipated for the
conductor while in use.
In many cases, an additional physical structure may be incorporated into
the jacket material or applied over it to impart improved physical
characteristics. For instance, wire, glass fibers, polymeric fibers, and
the like may be applied over a jacketed conductor to provide greater
longitudinal strength or other physical characteristics not provided by a
jacket alone. At other times, material may be combined to form a composite
structure to provide such properties (e.g., silicone coated glass fiber
braid, foil laminated polyesters, etc.).
Unfortunately, producing a correct balance between different properties may
be extremely difficult for many applications. For example, it is often
necessary to have a cable with a great deal of flexibility while being
sufficiently strong to resist elongation or breakage if longitudinal
strain is applied to the conductor. The use of a longitudinally strong
jacket to relieve such stresses on a conductor is commonly referred to as
"load sharing." Although it is a relatively simply matter to reinforce a
conductor to produce good stretch resistance, typically such reinforcement
significantly reduces other properties such as a wire's flexibility. With
the use of very fine conductors (e.g., on the order of less than 0.25 mm),
load sharing may be very important but must be balanced against loss of
the flexibility of such wires.
Achieving an acceptable balance between these properties is often not
possible. While a jacket of silicone has a high degree of flexibility, it
provides very poor resistance to elongation, thus contributing virtually
no load sharing to longitudinal forces. The reinforcement of such
materials with wires or fibers imparts the needed longitudinal strength,
but tends to simultaneously make the wire far less flexible.
Another concern often encountered is that many conductors are subjected to
extremely harsh environmental conditions that can weaken or destroy
conventional jacket materials. For instance, in medical applications it is
commonly necessary to sterilize cabling with steam at high temperatures
and pressures (e.g., in an autoclave) and/or with harsh chemicals. Many
otherwise suitable materials are incapable of withstanding such
treatments. Silicone and polyurethane are notoriously incapable of
withstanding high temperature treatments and may be subject to degradation
by certain chemicals. For example, silicone jacketed conductors tend to
expand significantly during steam sterilization, requiring the use of a
reinforcement material to avoid over-expansion and damage to the cable.
Porous, expanded PTFE, such as that made in accordance with U.S. Pat. No.
3,953,566 to Gore, has excellent dielectric properties and functions
extremely well as a cable jacket for most applications. Among its
desirable attributes are excellent strength and flexibility, high
temperature resistance, and chemical resistance. Unfortunately, the porous
nature of expanded PTFE may allow certain harsh chemicals (e.g.,
gluteraldehyde) to penetrate its interstices. At a minimum, this may
result in undesirable wetting of the expanded PTFE jacket material. At
worse, such chemicals can alter the properties of the jacket material
(e.g., making the jacket less flexible) or even diminish performance of
the cable by causing de-lamination or by attacking or interfering with the
conductor itself. This risk of conductor damage may be of particular
concern where exposure to such chemicals is combined with repeated high
temperature and pressure stream sterilization treatments in autoclave
cycling.
Another problem with a number of existing jacket materials is that they are
too often limited in their handling requirements. Even extremely effective
jacket material like expanded PTFE would be significantly improved if it
could be produced with certain improved elastic properties. An expanded
PTFE with improved axial elasticity may impart better abrasion resistance,
improved cut-through resistance, and more forgiving handling
characteristics.
It is accordingly a primary purpose of the present invention to provide a
jacket material for conductors that provides good load sharing along the
longitudinal length of a wire while contributing minimal resistance to
flex.
It is a further purpose of the present invention to provide a jacket
material that can withstand extremely rigorous use conditions, such as
sterilization procedures, without compromise of the jacket or the
conductor.
It is still another purpose of the present invention to provide a jacket
material with a variety of improved handling properties, increasing both
the range of possible uses for the jacket material and the ease of
applying and using such material.
These and other purposes of the present invention will become evident from
review of the following specification.
SUMMARY OF THE INVENTION
The present invention comprises an improved jacket composition for
protecting wires, optical fibers, and other conductors. The jacket
composition of the present invention is a composite of porous, expanded
polytetrafluoroethylene (PTFE) matrix permeated with a silicone polymer
that envelops the matrix structure of the expanded PTFE. Through the
selection of the expanded PTFE matrix, the type of silicone material, and
the method of application, the composite may be formed to be either
elastic or inelastic in its longitudinal dimension (imparting load sharing
where required) while retaining excellent axial flexibility through a
broad temperature range.
The resulting unique composition produces a jacket material that retains
some of the best characteristics of both expanded PTFE and silicone while
diminishing or eliminating some of the deficiencies of each, Of particular
interest is the ability of the jacket material to withstand harsh
environmental conditions, such as repeated chemical and/or steam
sterilization treatments, without degradation of either the jacket
material or the conductor(s) it contains. Further, the composition of the
present invention has a plethora of other improved handling qualities,
including abrasion resistance, cut-through resistance, selective strain
relief, selective elasticity, improved lubricity over elastomer alone, and
the ability to be made fully biocompatible.
DESCRIPTION OF THE DRAWINGS
The operation of the present invention should become apparent from the
following description when considered in conjunction with the accompanying
drawings, in which:
FIG. 1 is a scanning electron micrograph (SEM) of a composition of the
present invention, enlarged 2,000 times, showing the polymeric nodes and
fibrils structure coated with a low concentration of silicone material;
FIG. 2 is an SEM of a composition of the present invention, enlarged 2,000
times, showing the polymeric nodes and fibrils of the structure coated
with a medium concentration of silicone material;
FIG. 3 is an SEM of a composition of the present invention, enlarged 2,000
times, showing the polymeric nodes and fibrils of the structure coated
with a high concentration of silicone material;
FIG. 4 is a three-quarter isometric view of a multiple layered coaxial
cable including multi-stranded conductors, an insulative layer, a shield
layer, and a wrapped jacket material of the present invention;
FIG. 5 is a cross-section view of the cable along line 5--5 of FIG. 4;
FIG. 6 is a three-quarter isometric view of a conductor tape wrapped with a
jacket material of the present invention;
FIG. 7 is a three-quarter isometric view of a conductor sealed within a
continuous tube of jacket material of the present invention;
FIG. 8 is a three-quarter isometric view of a conductor "cigarette" wrapped
within a tape of jacket material of the present invention;
FIG. 9 is a side cross-section view of a multiple strand fiber optic cable
sealed within a jacket material of the present invention;
FIG. 10 is an end view of a multiple strand fiber optic cable sealed within
a jacket material of the present invention;
FIG. 11 is a graph showing flex test results on commercially available
samples of jacketed fiber-optic cable;
FIG. 12 is a graph showing flex test results on two samples of fiber optic
cables sealed within the jacket material of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is an improved protective jacket material suitable
for covering a wide variety of electro-magnetic energy conductive
materials, including electrically conductive cables transmitting
electrical signals and fiberoptic cables transmitting light signals. The
jacket of the present invention is particularly suitable for use in
protecting cables that must withstand harsh environmental conditions, such
as repeated steam sterilization treatments, or aggressive handling
demands, such as repeated flexure.
The protective jacket of the present invention comprises a composite of a
porous substrate of expanded polytetrafluoroethylene (PTFE) imbibed with a
silicone elastomer material. This composite may be formed in the following
manner.
First, an expanded PTFE material is produced, such as through the methods
described in US. Pat. No. 3,956,566 to Gore and U.S. Pat. No. 4,187,390 to
Gore, each incorporated by reference. For instance, an expanded PTFE tube
may be formed from a mixture of PTFE resin (having a crystallinity of
about 95% or above) and a liquid lubricant (e.g., a solvent of naphtha,
white oil, mineral spirits, or the like). The mixture is thoroughly
blended and then dried and formed into a pellet. The pellet is extruded
into a tube shape through a ram-type extruder. Subsequently, the lubricant
may then be removed through evaporation in an oven. The resulting tube
material may then be subjected to uniaxial or biaxial stretching at a
temperature of less than 327.degree. C. to impart the desired amount of
porosity and other properties to the tube. Stretching may be performed
through one or more steps, at amounts varying from 1:1 or less up to 45:1.
The resulting tube may then be subjected to a sintering temperature above
345.degree. C. (i.e., the melting temperature of PTFE) to amorphously lock
the tube in its expanded orientation.
Alternatively, a flat tape or membrane may be formed through a similar
procedure except that the dried pellet is extruded into a flat sheet. Once
expanded and amorphously locked, this sheet may then be cut into any
desired shape, such as a tape suitable for spiral or cigarette wrapping
around a conductor.
In both of these instances, a porous, expanded structure is obtained. As is
shown in FIG. 1, the expanded PTFE structure 10 comprises polymeric nodes
12 interconnected by fibrils 14. Typical properties of such a structure
comprise an average fibril length between nodes of 0.05 to 30 .mu.m
(preferably 0.2 to 10 .mu.m), and a void volume of 20 to 70% (preferably
30 to 50%). As should be evident from the following description, the
precise properties and dimension of expanded PTFE structures employed with
the present invention are a function of application. The general membrane
properties suitable for use with the present invention should include
medium to high porosity, and wettability by various solvents, such as
methylene chloride, toluene, and/or acetone.
The fibril length of expanded PTFE that has been expanded in single
direction is defined herein as the average of ten measurements between
nodes connected by fibrils in the direction of expansion. The ten
measurements are made on a representative micrograph of an expanded PTFE
sample. The magnification of the micrograph should be sufficient to show
at least five sequential fibrils within the length of the micrograph. Two
parallel lines are drawn across the length of the micrograph so as to
divide the image into three equal areas, with the lines being drawn in the
direction of expansion and parallel to the direction of orientation of the
fibrils. Measuring from left to right, five measurements of fibril length
are made along the top line in the micrograph beginning with the first
nodes to intersect the line near the left edge of the micrograph, and
continuing with consecutive nodes intersecting the line. Five more
measurements are made along the other line from right to left, beginning
with the first node to intersect the line on the right side of the
micrograph. The ten measurements obtained by this method are averaged to
obtain the average fibril length of the material.
Substrate material made through one of the above described methods and
suitable for use in the present invention is commercially available in a
wide variety of forms from a number of sources, including under the
trademark GORE-TEX.RTM. from W. L. Gore & Associates, Inc., Newark, Del.
Once suitable porous, expanded PTFE substrate material is obtained, the
following processing is performed to produce the composite material of the
present invention. First a solution is formed by dissolving a silicone,
such as a fluoro-silicone, in an organic solvent. The ratio of silicone to
solvent should be in the range of 4:1 to 1:10 parts by volume, and
preferably is in the range of 3:1 to 1:3 parts by volume. The solution is
formed through any conventional means, such as by blending in a mechanical
mixer under ambient conditions. Where high loading of silicone is desired,
elevated temperatures may be employed below the boiling temperatures of
the solvent.
The preferred solutions comprise a silicone material comprising a material
soluble in one or more solvents capable of permeating and wetting out an
expanded PTFE structure. The material preferably has a solids content of
95-100%, a specific gravity of between 0.95 to 1.5, and a viscosity
between 300 and 150,000 centipoise. The material is preferably translucent
in color. Further, the material preferably employs a one or two part cure
system, ideally at an elevated temperature, to cure the liquid silicone
into a rubber-like mass. It is particularly preferred to use a silicone
with a platinum-type cure system that is activated at elevated
temperatures to cross-link into a rubber-like substance.
It is suggested to select the silicone material from the group consisting
of siloxane or polysiloxane having reactive groups, alkoxysilane or
partially hydrolyzed forms thereof, and copolymeric siloxane having
reactive groups. Known curing silicone rubber material compositions
include normal temperature curing types, low temperature curing types, and
high temperature curing types. Suitable silicones for use in the present
invention include methylhydrogen siloxane, dimethylhydrogen siloxane,
dimethyl siloxane, dimethylvinyl-terminated siloxane,
dimethylmethylphenylmethoxy silicone polymer, and the like. Additionally,
the silicone can contain dimethylvinylated silica, trimethylated silica,
and the like. Commercially available silicone for use with the present
invention include Q3-6611, X1-4105, and Q1-4010, all available from Dow
Coming, Corp., Midland, Mich.
Room temperature curing and high temperature curing compositions of
silicone include two-pack types materials. Two-pack type materials deliver
a silicone rubber having cross-linked structure by means of a reaction
between siloxanes having reactive groups (e.g., SiOH, SiO-R (where R is an
alkyl group), SiH, SiCH.dbd.CH.sub.2 or the like) in the presence of a
catalyst. The two-pack compositions are divided into condensation reaction
types and addition reaction types.
The condensation reaction types include those employing:
dehydration-condensation reactions between silanol and alkoxy siloxane; a
de-alcoholation condensation reaction between silonal and alkoxy siloxane;
and a dehydrogenation condensation reaction between SiH and silanol. The
addition reaction types include those employing addition reaction between
vinyl groups, alkyl groups, or other unsaturated groups and SiH.
A suitable curing catalyst is selected depending on the type of curing
reaction desired. For example, metal, organic-metal salts, organic amines,
quaternary ammonium salts, and the like are employed in reactions of
condensation reactions types. Palladium or platinum back, platinum
asbestos, chlorplatinic acid or other form of platinum are employed in
reactions of addition reaction types. The above-mentioned compositions may
also contain other materials, such as silicone oil, SiO.sub.2, or fumed
silica as property altering agents.
The preferred solvent comprises a solvent that both actively dissolves the
silicone and is readily absorbed into structure of the intended polymeric
substrate. For use with a PTFE substrate structure, a halogenated solvent,
such as methylene chloride, acetone, or toluene, is particularly useful,
as are commercially available solvents NORPAR-12 and ISOPAR-C. While
methylene chloride has produced the best results to date, the carcinogenic
nature of this solvent is objectionable for some applications.
Accordingly, other preferred solvents continue to be sought.
The presently preferred composition comprises a mixture of 10-75% by weight
of Q1-4010 silicone elastomer and 25-90% methylene chloride, acetone, or
toluene solvent. This mixture is formed by stirring the solvent while
adding the silicone elastomer at room temperature (about 22.degree. C.)
until the mixture has achieved a homogenous color. With an acetone
mixture, the mixture should be re-stirred prior to each use due to
precipitation of materials.
Once the silicone/solvent composition is formed, it can then be applied to
any suitable microporous membrane. The preferred membrane for use with the
present invention comprises the porous expanded PTFE material described
above. Another membrane material which may be suitable for use with the
present invention is expanded ultra-high molecular weight polyethylene and
perhaps others that can be expanded into an open, porous network of nodes
and fibrils.
The solution is applied to the porous PTFE material by spreading the
composition evenly over the membrane and then allowing the composition to
become absorbed therein. Preferably, the PTFE material is immersed within
the solvent until it becomes saturated, such as by submerging the material
in a bath of solution over a period of 1 to 5 minutes. The solution may be
placed under reduced pressure, such as in a vacuum chamber, to facilitate
complete filling of the porous polymeric substrate.
Once filled, the membrane and absorbed solution is exposed to an energy
source, such as a heated oven set at 70.degree. to 75.degree. C. or above,
for a period of 2 to 5 minutes or more to evaporate away any solvent.
Ideally, evaporation comprises employing an oven heated to 85.degree. C.
or above and exposing the composition for at least 5 minutes. The
evaporation of solvent can also be performed in one of the following
manners: air drying for about a 5 hour period; or about 1 hour at about
50.degree. C. in an explosion-proof oven.
When applied in this manner, it has been found that the porous PTFE
material will become thoroughly impregnated with the silicone between its
top and bottom surfaces. When applied to a flat membrane by spreading on
one side, the bottom surface of the membrane (i.e., the surface opposite
the side where the composition is applied) tends to have a tacky feel to
it that may be desirable if the membrane is to be used as an adhesive
layer.
By contrast, with some applications the top surface of the membrane has
been found to have a powder-like material on it. This is believed to be a
coating of silicon dioxide found as a filler in some commercial silicone
materials. This material may be left in place for ease in handling or may
be removed through any suitable means, such as through use of a solvent
and/or mechanical scraping. Additionally, it may be possible to adjust the
pore size of the membrane to allow the infiltration of filler or
extraneous material into the membrane along with the silicone. Different
silicone mixtures, both with and without silicone dioxide filler, are
described in the examples set forth herein.
After impregnation, the composite material may then be subjected to
appropriate conditions to cure the silicone material. For a Q1-4010 type
silicone of Dow Corning Corp., a filled PTFE membrane can be cured by
placing the composite material within an oven at about 110.degree. C. for
about 30 minutes.
The goal of the present invention is to provide a complete overlay of
silicone over the polymeric nodes and fibrils of the membrane. Depending
upon the conditions employed, the complete impregnation of the membrane
may comprise simply covering the polymeric structure while leaving the
microporous structure open to air permeation. Alternatively, the entire
fibrillated interior of the membrane, including most or all of the porous
structure therein, can be filled with the silicone.
FIG. 2 shows the fibrillated PTFE structure, such as that shown in FIG. 1,
with a medium coating of silicone polymer coating both the nodes and
fibril structure of the polymer to form the composite of the present
invention. The material was the result of placing an expanded PTFE
membrane in a solution of 50% by volume of Q1-4010 silicone and a 50% by
volume of ISOPAR-C solvent for 1 minute. This filled material was then
heated in an oven for 10 minutes at about 110.degree. C. to achieve a
final product.
FIG. 3 shows a fibrillated PTFE structure, such as that shown in FIG. 1,
with a heavy coating of silicone polymer coating both the nodes and fibril
structure of the polymer to form the composite of the present invention.
As can be seen, the node and fibril structure of this material is
thoroughly loaded with silicone. The material was the result of placing an
expanded PTFE membrane in a solution of 75% by volume of Q1-4010 silicone
and a 25% by weight of ISOPAR-C solvent for 1 minute. This filled material
was then heated in an oven for 10 minutes at 110.degree. C. to achieve a
final product.
In either instance, the intent of the present invention is to produce a
thoroughly impregnated composite membrane that has substantial elastomeric
properties. In this respect, the degree of elasticity of the present
invention can be measured in the following manner: a piece of treated and
cured membrane is measured in length, stretched 2 times its length,
released, and its new length re-measured. Resiliency is measured by
compressing a given thickness of treated membrane to 50% its original
height for 1 minute, releasing, and re-measuring its thickness.
To further aid in the impregnation process, the process of the present
invention may be combined with other processes to achieve specific
properties. For example, for some applications, such as use with very fine
porous membranes, it may be desirable to impregnate the membrane with the
silicone/solvent composition with the aid of a mechanical vacuum process.
Other possible methods include use of mechanical pressure through either a
pressure or vacuum process.
Once formed in this manner, the composite of the present invention may then
be formed into conductor jacketing. FIGS. 4 and 5 illustrate one example
of a wire 16 protected with a jacket 18 of the present invention. In this
instance, the wire 16 is a coaxial cable comprising a multi-strand center
conductor 20, enwrapped in a dielectric material 22, in turn wrapped in a
braided wire or foil shield 24. In order to protect these functional
elements of the coaxial cable, the entire structure is then wrapped in a
protective jacket 26. The jacket 26 illustrated comprises a tape formed
from a composite membrane of the present invention that has been spiral
wrapped around the shield 24. The jacket 26 may be held in place through
any conventional mean, such as with adhesive applied between the jacket
and the shield. Alternatively, the jacket 26 may be bonded to the shield
24 by wrapping the jacket on the cable prior to curing the silicone and
then heating the assembled wire to the curing temperature of the silicone,
causing the silicone to flow and adhere to the braid and itself.
This construction has proven to be very effective at protecting coaxial
cables and the like. While use of silicone material alone as a jacket is
desirable for its flexibility, it tends to have very poor resistance to
elongation. This means that the material housed by the jacket must bear
load place on portions of the jacket alone, with no "load sharing"
proffered by the jacket. However, the composite material of the present
invention retains the flexibility of silicone while significantly
strengthening it to allow for substantial load sharing properties. In the
case of coaxial cables and the like, this translates into more consistent
signal transmission over the length of the cable, with less likelihood
distortion occurring when portions of the cable are placed under load.
Another advantage of a cable jacket made in accordance with the present
invention is its ability to withstand harsh environmental conditions. As
was previously explained, certain jacket materials such as silicone and
polyurethane are known to have limited ability to withstand sterilization
techniques or exposure to harsh chemicals. High temperature steam
treatments will rapidly degrade silicone wire jackets, diminishing their
flexibility and compromising their protective properties. The composite of
the present invention is not so constrained. It has been found that
expanded PTFE effectively protects the silicone material during steam
sterilization procedures so that the material can withstand many more
autoclave cycles with no impact on its performance. This makes the jacket
material of the present invention particularly suitable for use with
medical apparatus and other devices that experience repeated exposure to
harsh environmental conditions.
Another construction of the present invention is shown in FIG. 6. In this
instance, a cable 26 comprises merely a center conductor 28 spiral wrapped
with a composite tape jacket 30 of the present invention. This
construction is suitable for a wide variety of applications, from mere
metal electrical conductors to advanced fiber-optic cable construction.
Again, this construction provides significant improvements in the handling
characteristic of the wire, including improved load sharing and ability to
be used under harsh conditions.
The construction shown in FIG. 7 illustrates another suitable method for
sealing virtually any form of conductor 32. In this case, a jacket 34 is
formed from a continuous tube of composite material of the present
invention. An expanded PTFE tube may be coextruded or otherwise formed
over the conductor and then filled with silicone in place, or the
composite tube may be formed in accordance with the present invention and
then drawn over the conductor.
Another effective means of wrapping a conductor 36 with a jacket 38 of the
present invention is shown in FIG. 8. Here a relatively wide tape is
wrapped longitudinally around the conductor in a "cigarette" wrap fashion.
A single seam 40 is formed down the length of the conductor and may be
held in place through any of the adhesion methods previously described.
One area of particularly growing interest for the composite of the present
invention is as a jacketing for single or multiple arrays of fiber-optic
conductors. One construction for use in this area is shown in FIGS. 10 and
11. A fiber-optic cable assembly 42 is shown comprising a multiple strand
fiber-optic conductor 44 wrapped in a tube of composite jacket material 46
of the present invention. Each end of the jacket 46 is terminated with
stainless steel end connectors 48, 50 that are glued in place with epoxy
adhesive. As is explained in greater detail below, this construction has
produced dramatic improvement in flexure resistance of these cables.
Two of the particularly important aspects of the present invention are the
improved properties it provides in both resistance to compression and in
its ability to repeatedly withstand high temperature treatments without
degradation. With regard to compression resistance (or "resilence"), the
wire jacket of the present invention will withstand compression in its
axial dimension of 75% with a rebound of at least 50% to its original
dimensions. This is a notable improvement over many presently available
materials, such as conventional expanded PTFE, which have minimal crush
resistance. With regard to the temperature resistance of the present
invention, it has the ability to withstand repeated exposure to
temperature in excess of 250.degree. C. without significant degradation.
This property makes it particularly suitable for demanding applications,
such as uses demanding repeated sterilization.
A further improvement of the jacket material of the present invention can
be achieved by filling the expanded PTFE structure with particles or
material which enhance one or more properties. For instance, the jacket
material may include conductive shielding properties by including
electrically conductive particles within the expanded PTFE. In a preferred
embodiment, the jacket material may include particles, fibers, or other
fillers of one or more of the following: carbon, graphite, aluminum,
silver plated aluminum, copper, copper alloy, iron, iron alloy, nickel,
cobalt, gold, silver or silver plated copper, or the like. Filler content
preferably comprises 5-85% by volume of the PTFE/filler composition. A
preferable composition of PTFE filler for use as a jacketing tape includes
the filler comprising 30 to 50% by volume of the mixture. Material made in
this manner will also provide shielding against electrical and
electromagnetic effects.
Alternatively, the jacket may be constructed of expanded PTFE that has been
plated with a metal. Such a plating process is described in U.S. Pat. No.
4,557,957 and U.S. Pat. No. 4,720,400 both to Manniso, incorporated by
reference. Preferred metal plating materials include silver, silver-copper
alloys, gold, cobalt, platinum and copper alloys and most preferably
copper, nickel, and tin.
Without intending to limit the scope of the present invention, the
following examples illustrate how the present invention may be made and
used:
EXAMPLE 1
A composite membrane suitable for use as a jacket of the present invention
was produced in the following manner. A silicone adhesive material was
acquired from Dow Coming Corp., of Midland, Mich., under the designation
Q3-6611. The Q3-6611 contains dimethyl, methylhydrogen siloxane copolymer,
dimethylvinylated and trimethyled silica and quartz. This material
comprises one part gray colored thick flowable liquid having a viscosity
of 95,000 centipoise. After curing, but before impregnation in an expanded
PTFE structure, the material has a durometer measurement of Shore A 60,
tensile strength of 700 psi, and an elongation of 125%.
The silicone material was mixed in a halogenated solvent of methylene
chloride (an aqueous solution of 50% by weight). Mixing was performed by
stirring the solution at room temperature until a homogenous color formed.
The composite material of each of the mixtures was applied to sample
membranes of expanded PTFE made in accordance with U.S. Pat. No. 3,953,566
to Gore, incorporated by reference.
Coating was performed by using a wheel transfer procedure whereby a 25 foot
long by 6 inch wide by 0.008 inch thick piece of unsintered expanded PTFE
material was transported via a pay-off and take up reel system over a
rotating drum partially submersed in the silicone solvent mixture. The
drum was rotated in a direction opposite the direction of material travel.
A blade was positioned after the drum such that as the impregnated
material travels across it, excess silicone material was scraped off.
The extent of silicone penetration was unexpected and caused the uncoated
side of the membrane to become sticky. This stickiness caused the membrane
to drag and stick to the plenum that the membrane travels across for
drying of the solvent before being reeled up. Twenty (20) feet of
impregnated membrane was produced using this procedure.
Once coated in this manner, the membrane was cured for ten (10) minutes in
an oven at 150.degree. C. The pieces of membrane had very good elastic
properties and did not show the usual cold flow characteristics of
silicone.
Inspection of this membrane revealed that the side to which the silicone
had been applied had a thin layer of powder-like material on it. This is
believed to be a silicone dioxide deposit, which is a filler material
found in the Q3-6611 silicone. The other side of the membrane had a sticky
or tacky feeling indicating that the silicone completely penetrated the
membrane. The essentially opaque white membrane also became translucent
through this penetration.
In order to establish adhesive qualities on both sides of this membrane,
the powder layer was removed from some of the samples by scraping the
uncured membrane using a blade.
In order to demonstrate one application of the present invention, the
sheets produced were placed in alternating layers with flexible printed
circuit board material. This composite was then placed in a heated press
at 500 lbs at 150.degree. C. for ten minutes. The resulting flexible
composite had excellent adhesion between the layers and some elastic
properties.
EXAMPLE 2
Another mixture was created employing a silicone without a filler material.
This material was a Dow Corning Q1-4010 Silicone Conformal Coating. The
Q1-4010 contains dimethyl, methylhydrogen siloxane copolymer, dimethyl
siloxane, dimethylvinyl-terminated silica and trimethylated silica. The
silicone was thinned using a solvent of methylene chloride. Mixing was
again accomplished by stirring at room temperature until a homogenous
color is produced.
Instead of a wheel coater machine, application to a membrane material
identical to that employed in Example 1 was performed by placing the
membrane material on a layer of silicone release paper. A 50:50 by weight
mixture of silicone and methylene chloride was poured onto the membrane
and wiped across the entire surface until a uniform translucence was
achieved.
This same procedure was performed on a number of expanded PTFE membrane
samples. Each of the samples was then cured for 10 minutes at 150.degree.
C.
EXAMPLE 3
A tubular composite material of the present invention was produce in the
following manner and was tested as a wire jacket in the manner described
below.
A dispersion of polytetrafluoroethylene was prepared in the following
manner. A PTFE resin acquired from E. I. duPont de Nemours and Company was
mixed with 17% by volume lubricant of ISOPAR-C. This mixture was
compressed into a pellet at 200 psi. The pellet was then extruded through
a ram-type extruder at 300 psi to form an extruded tube. The extruded tube
was dried at about 300.degree. C. for about 5 seconds. The tube was then
expanded 4:1 at a line speed of 24 feet/min. The expanded tube was then
sintered at 395.degree. C. for 17 seconds. The resulting tube had a
density of about 0.5 g/cc and a nominal wall thickness of about 0.030
inches.
By placing a wire assembly within the above described tube, a composite
wire jacket of the present invention was produced in the following manner.
The tube and wire assembly were submerged in a solution of silicone and
solvent (ISOPAR-C) per the table below. One sample of each type cable was
obtained for testing. The seven samples are described as follows:
______________________________________
% Loading Silicone
Samples No.
of Silicone Resin No. P.R. No.
______________________________________
1 25 Q1-4010 N/A
2 50 Q1-4010 N/A
3 75 Q1-4010 N/A
4 25 X1-4105 N/A
5 50 X1-4105 N/A
6 100 Q7-4750 83-W1036-04
7 0 N/A N/A
______________________________________
Each of these samples were then tested using the following equipment:
Instron 4201 Universal Tester with 1000 lb. Tensile Load Cell
100 lb. Compression Load Cell
0.003" Radius, 0.250" Wide Blade Fixture
Delron Insulation Spacer
Short Detector
Teledyne Taber Model V-5 Stiffness Tester
90.degree. Tik Tok Tester
Pelton-Crane Validator Plus Autoclave
The cables were evaluated using the following procedures.
1. The test samples were tested for stiffness using the Taber Model V-5
Stiffness Tester and following the manufacturers recommended procedure.
2. The test samples jackets were removed, dimensions taken, and tensile
strength testing performed using the Instron 4201 Universal Tester with
the following procedure:
(a) Cut sample to be tested into 6 inch lengths and remove the sample core;
(b) Obtain overall OD, wall thickness, and jacket ID by measurement of
jacket material;
(c) Set-up Instron 4201 using the manufacturers recommended procedures with
the 1000 lb. load cell and allow to stabilize;
(d) Set the crosshead speed at 2 inches per minute and the sample clamps at
1 inch apart;
(e) Install the sample to be tested into the clamps and pull;
(f) Make several sample pulls, record the "Peak" values for each pull, then
average the values for an average peak force;
(g) Using the equation below, compute the average PSI for the sample being
tested and record:
##EQU1##
3. The test sample jackets were tested for cut-through resistance using the
Instron 4201 Universal Tester and the following the Requesting &
Performing Cable Crush Test Procedure, Document No. 06-00021-01, in the
following manner:
Particular parameters:
100 lb. Compression Cell
0.05 in/min.
0.003" Radius, 0.250"Wide Blade Fixture
Procedures:
(a) Cut sample to be tested into 6 inch lengths and remove the sample core;
(b) Note overall OD, Wall Thickness, and jacket ID measured in previous
test;
(c) Set-up Instron 4201 using the manufacturers recommended procedures with
the 100 lb. compression load cell and allow to stabilize:
(d) Set the crosshead speed at 0.05 inches per minute and install the
delron spacer and the 0.003" radius blade fixture;
(e) Install the sample to be tested onto the 0.003" radius, 0.250" wide
blade fixture and attach the short detector, red lead to the load cell and
the black lead to the fixture;
(f) Make several sample compressions, stopping the crosshead when the short
detector detects a short and record the "Peak" value for each compression.
Average the values for an average peak force and record.
4. The test samples were installed on the 90.degree. Tik Tok Tester and
flexed following the Requesting & Performing Cable Flex Test Procedure,
Document No. 06-00034-01 in the manner described below, to evaluate the
jacket material resistance to flexing. The set up for the test was as
follows:
Mandrel OD--0.320"
Flex Rate--15 cycles per minute
Tension--1 lb.
Cycles Completed 496,653
Sample evaluation--No failure of jackets, no cracks, no splits, etc., all
samples
Electrical testing was not performed during the flexing since only jacket
evaluation was desired.
The procedure for the test was as follows:
(a) The sample material was mounted in a Tik Tok Tester;
(b) The sample was then exposed to repeated 90.degree. folds, in opposite
directions, over the mandrel at the stated flex rate;
(c) The samples were visually evaluated upon completion of the designated
number of cycles.
5. One sample of No. 5 was autoclaved using the Pelton-Crane Validator Plus
Autoclave and following the manufacturers recommended procedures. The
individual cycle definitions were as follows:
Temperature 270.degree.-274.degree. F.
Pressure 30-38 PSI
Duration 5 minutes
Tensile strength and cut-through tests were performed, as described above,
after autoclaving to determine any material property changes.
6. Samples 6 & 7 are control samples, with No. 6 being a 100% silicone
jacket material and No. 7 being a conventional 100% expanded PTFE tube
mounted as a jacket over a wire assembly.
7. The samples were tested for crush resistance in the following manner:
(a) A sample of cable was prepared for electronic monitoring, with all
center conductors connected together and attached to approximately an 18
inch lead. All shields were connected together and were also attached to
an 18 inch lead. The opposite end of the test sample was exposed and the
wires separated to prevent accidental shorting;
(b) An INSTRON 4210 Universal Tester was employed having a 100 lb.
compression load cell with a flat plate installed. A 0.003" radius Blade
Fixture was installed on the Universal Tester;
(c) The leads from the test sample was attached to a SLAUGHTER Series
103/105-MP Hi-Pot Tester;
(d) The test sample was positioned over and perpendicular to the 0.003"
radius blade. The blade was positioned at one end of the sample so as to
allow it to work its way up the sample;
(e) The compression cell was positioned to just touch the test sample, with
crosshead speed set at less than 1.0 inch per minute;
(f) The Hi-Pot tester was set at 500 V DC with a current limit of 10 mA;
(g) The Universal Tester compressions cycle was then initiated while
watching for a short in the sample. When a short is indicated, readings
were taken.
8. Flex testing was performed in accordance with the following procedure:
(a) A sample was prepared for electrical monitoring, with a daisychain of
conductors connected together and attached to approximately 18 inch leads
at each end;
(b) One end of the sample was installed into a clamp in a 180.degree. Cork
Screw Flex Tester;
(c) The sample was then twisted three rotations (counterclockwise) and then
connected to the opposite clamp;
(d) The sample was connected to a 200/50 Point Break Detect Box;
(e) The tester was run at 15.+-.1 cycles per minute until failure occurred;
(f) Upon failure, the number of cycles to failure were noted.
The final results of the above tests are summarized below.
__________________________________________________________________________
Post Autoclave
Post
Test
Taber
Tensile
Cut- Tensile Autoclave
Sample
Stiffness
Strength
Through Strength
Cut-Through
No. (gm-cm)
(PSI)
(lbs)
Flex Test
Test (PSI)
Test (lbs)
__________________________________________________________________________
1 66.7 7565 33.67
No Failure
N/A N/A
2 79.2 7577 34.45
No Failure
N/A N/A
3 89.2 8363 32.19
No Failure
N/A N/A
4 71.7 7368 31.42
No Failure
N/A N/A
5 89.2 5359 32.77
No Failure
7783 PSI
42.0
6 115.0
1385 22.45
Surface wear
N/A N/A
7 56.25
8012 47.57
No Failure
N/A N/A
__________________________________________________________________________
The following conclusions were reached with regard to the above testing:
Taber Stiffness Tests
The Taber Stiffness test is an indication of the flexibility of the test
sample. The higher the Taber number, the stiffer the sample. The value is
a relative reference in gauging stiffness of one test sample with another.
Test sample No. 6 (100% silicone jacket) data was obtained from test data
taken from previous production lots. This is the value used in test for
customer acceptance.
The test indicated a trend within the two resin types, with an increase in
stiffness developing as the percent of loading increases. The samples Nos.
1, 2 & 3 (Q1-4010 resin) indicated a slightly higher level of stiffness at
the same percent loading compared to the samples Nos. 4 & 5 (X1-4105
resin).
Comparing the samples to the control samples, all samples were more
flexible than the silicone control and less flexible than the ePTFE
jacketed control.
Tensile Strength Tests
The tensile strength test also indicated a similar trend as the Taber test
illustrated but with one slight deviation. The samples Nos. 1, 2, & 3
(Q1-4010 resin) increase, but sample No. 5 (X1-4105 resin) does not show
the increase expected. No explanation noted. The relationship between the
control samples Nos. 6 & 7 also slightly different with the sample No. 3
being higher than the sample No. 7. No explanation noted. The minimum
value for test sample No. 6 is 1200 PSI per the suppliers data sheet for
the Q7-4750 material.
Cut-Through Tests
The cut-through test was performed using the 0.003" radius, 0.250" wide
blade fixture at a crosshead speed of 0.05"/minute and recording the
maximum force required to penetrate one thickness of the jacket material.
Tests conducted on different configurations indicated that the thickness
of the sample had no effect on the maximum force required to penetrate the
material, it only affected the time duration before the short was noted.
Flex Tests
The flex test results showed no failures. No cracks, splits, or wrinkles
were noted. The only results noted was the normal jacket discoloration
found during all flex tests and this is attributed to the use of aluminum
mandrels. Conventional silicone jacket showed signs of surface wear.
Autoclave Tests
The post autoclave tensile strength and cut-through tests were performed,
as described above, on the only test sample that was autoclaved. This test
sample was noted to be No. 5. The results indicated that the tensile
strength and cut-through were increased due to autoclaving.
The following table summarizes testing results on samples produced above
and tradition silicone jacket over a multi conductor core of silver plated
copper conductors.
__________________________________________________________________________
POST AUTO-
POST AUTO-
TABER TENSILE
CUT CLAVE CLAVE CUT
RATIO Si STIFFNESS
STRENGTH
THROUGH
TENSILE THROUGH
Si:SOLV
MATERIAL
(g/cm) (PSI) (lbs) (PSI) (lbs)
__________________________________________________________________________
1:3 Q1-4010
66.7 7565 33.67 N/A N/A
1:1 Q1-4010
79.2 7577 34.45 N/A N/A
3:1 Q1-4010
89.2 8363 32.19 N/A N/A
1:3 X1-4105
71.7 7368 31.42 N/A N/A
1:1 X1-4105
89.2 5359 32.77 7783 42.0
Si ONLY
SEE NOTE
115.0 1385 22.45 N/A N/A
__________________________________________________________________________
This example provided over 12 times the tensile strength of a conventional
silicone jacket and was inelastic in the longitudinal axis. This
characteristic allows this invention to provide for longitudinal strain
relief while remaining extremely flexible. With conventional silicone
jacket this longitudinal strain relief must be provided by wire or fiber
braid or stranding.
EXAMPLE 4
A flat film of the present invention was produced in the following manner.
A dispersion was produced using a mixture of 145 cc of ISOPAR-K per pound
of duPont T-3512 resin. The mixture was extruded using a Gore double
cavity die in a ram-type extruder. The extruded material was then
calendered from 0.026" thick to 0.006" thick. This material was then
calendered again to 0.0042" thick. The resulting material was then dried.
The dried, calendered material was then expanded at a rate of 3.55:1
nominal expansion rate at a line speed of 130 feet/min. Finally, the
expanded material was sintered at a temperature of about 369.degree. C.
The resulting material had the following properties: 0.75 gm/cc density;
70% porosity; 12 psi bubble point; 10,610 psi matrix tensile strength in
the longitudinal axis of the membrane; 3.55:1 nominal expansion ratio;
2,735 psi matrix tensile strength in the transverse axis of the membrane;
and an inverted cup moisture vapor transmission rate (MVTR) of 8300.
These membrane properties were determined in the following manner:
Density was determined by measuring the dimensions of the material and
calculating its weight per unit area.
Porosity was determined by calculating the sample's intrinsic density using
a Micromeritics Model 1310 autotychometer. The procedure followed was: to
evacuate the sample of air by using helium by exposing the sample to a
helium-filled environment for 5 minutes; to determine the bulk density of
the sample through an Archimedes method of water displacement of a
1".times.1" sample; and to calculate the porosity in accordance with the
following calculation:
Porosity=1-((bulk density)/(intrinsic density)).times.100%.
The Bubble Point of porous PTFE was measured using isopropyl alcohol
following ASTM Standard F316-86. The Bubble Point is the pressure of air
required to blow the first continuous bubbles detectable by the their rise
through a layer of isopropyl alcohol covering a 1 inch circular sample.
This measurement provides an estimation of maximum pore size.
The tensile strength was determined in accordance with ASTM D-882 (Tensile
Properties of Thin Plastic Sheeting) using an Instron Tensile Tester,
Series IX. The cross-head speed of the tensile tester was set at about 20
inches/min and the gage length was set at 2 inches.
The Moisture Vapor Transmission Rate (MVTR) was determined by mixing
approximately 70 ml of a solution consisting of 35 parts by weight of
potassium acetate and 15 parts by weight of distilled water and placing it
into a 133 ml polypropylene cup, having an inside diameter of 6.5 cm at
its mouth. An expanded polytetrafluoroethylene (PTFE) membrane having a
minimum MVTR of approximately 85,000 g/m.sup.2 /24 hrs. (as tested by the
method described in U.S. Pat. No. 4,862,730 to Crosby) and available from
W. L. Gore & Associates, Inc. of Newark, Del., was heat sealed to the lip
of the cup to create a taut, leak proof, microporous barrier containing
the solution. A similar expanded PTFE membrane was mounted to the surface
of a water bath. The water bath assembly was controlled at 23.degree. C.
plus 0.2.degree. C., utilizing a temperature controlled room and a water
circulating bath.
The sample to be tested was allowed to condition at a temperature of
23.degree. C. and a relative humidity of 50% prior to performing the test
procedure. Samples were placed so the microporous polymeric membrane was
in contact with the expanded polytetrafluoroethylene membrane mounted to
the surface of the water bath and allowed to equilibrate for at least 15
minutes prior to the introduction of the cup assembly.
The cup assembly was weighed to the nearest 1/1000 g and was placed in an
inverted manner onto the center of the test sample. Water transport was
provided by the driving force between the water in the water bath and the
saturated salt solution providing water flux by diffusion in that
direction. The sample was tested for 5 minutes and the cup assembly was
then removed, weighed again within 1/10009. The MVTR of the sample was
calculated from the weight gain of the cup assembly and was expressed in
grams of water per square meter of sample surface area per 24 hours.
The expanded PTFE sheet made as described above was then submersed in a
mixture of 3:1 by volume of Dow Coming Q1-4010 silicone and ISOPAR-C
solvent in the manner described above using a wiper ("doctor") blade to
remove excess silicone from the surface of the membrane.
The initial membrane had the following properties:
After treatment in a 3:1 mix:
The composite material thus produced was then spiral wrapped around a
coaxial cable, such as in the construction shown in FIGS. 4 and 5. Once
wrapped, the wrapped cable was place in an oven at 110.degree. C. for 15
minutes to cure the silicone material and bond the composite jacket to the
coaxial cable.
After curing, the material forms a predominately homogeneous jacket over
the conductor assembly. This construction method allows for the
manufacture of an extremely thin jacket in the range of 0.0005 inches to
0.100 inches in wall thickness. Testing of this jacket showed
significantly improved properties over either silicone or expanded PTFE
jackets alone, including: better cut-through resistance; better abrasion
resistance; and improved lubricity over silicone jacket material. The
final material was tested to have the following properties: a matrix
tensile strength of 6068 psi in the longitudinal direction and 2048 psi in
the longitudinal direction; a density of 1.253; and a MVTR of 203.
The presence of expanded PTFE establishes a radial constraint on the
conductor assembly. This radial constraint is particularly useful in the
manufacture of high frequency coaxial transmission lines as it imparts
electrical loss stability to the transmission line.
EXAMPLE 5
The following examples that further processing of the composite material of
the present invention may be employed to achieve even more improved
properties.
A three foot tube composite was manufactured in accordance with Example 3,
above. Each end of the tube and including 2 inches extending inward from
each end was submerged in a solution of 1:1 parts by volume of Dow Coming
Q1-4010 silicone and ISOPAR-C solvent. The tube ends were removed from the
solution and placed in an oven at 120.degree. C. for 30 minutes to cure
the silicone. The tube was then submerged in its entirety in the solution
and allowed to permeate completely. Upon permeation the tube was removed
from the solution and placed in the oven as above for 30 minutes.
The resulting tube in this example had a more rigid area at either end of
the tube than in the center. This technique may be employed to produce a
tube with increased rigidity at any desired location or locations along
its length. This allows for enhanced strain relief of a transmission line
placed into the tube and terminated with connectors or clamped in the area
of increased rigidity.
EXAMPLE 6
A composite tube of the present invention was made in accordance with the
method described in Example 3, above, except with an expansion ratio of
5:1 and a line speed of 7.5 feet/min. The resulting tube had a wall
thickness of 0.030 inches.
The extruded tube was submerged in a 1:3 solution of Dow Corning Q1-4010
silicone and ISOPAR-C solvent. The tube was then removed and wrapped onto
a mandrel being held at each end of the tube. The tube and mandrel were
then place in an oven for 30 minutes at 110.degree. C.
The final composite jacket had an outside diameter (O.D.) of 0.300 inches
and an inside diameter (I.D.) of 0.180 inches.
A bundle of optical fibers was then pulled through this composite jacket
with the aid of a guide wire. The optical fiber bundle had an O.D. of
0.137" and each fiber had an O.D. of 0.002 inches. The optical fibers were
made of borosilcate glass material.
Each end of the cable was terminated by inserting the fiber bundle into a
stainless steel end connector. Epoxy adhesive was used to glue the fiber
bundles to the end connectors and to each other. Finally, the end faces
(i.e., cross-section of end connector containing the fiber optic bundle)
were polished of optical transmission measurement.
Two cable assemblies were manufactured in this manner. The cables were
tested in a flex tester according to EIA standard FOTP 104. For
comparison, two commercially available cables were also tested. The
commercial cables contained straight fiber bundles in a conventional
silicone jacket.
The test results for these cables are reflected in the graphs of FIGS. 11
and 12. FIG. 11 illustrates the performance of the commercially available
cables. This graph demonstrates that failure of these cables occurred
rapidly, with five cables failing after only 30,000 cycles of flexure, and
another cable failing after only 60,000 cycles of flexure.
By contrast, FIG. 12 illustrates the performance of the two cables made in
accordance with the present example. As can be seen, one of these two
cables failed after 430,000 cycles of flexure and one continued to show no
degradation at that stage.
While particular embodiments of the present invention have been illustrated
and described herein, the present invention should not be limited to such
illustrations and descriptions. It should be apparent that changes and
modifications may be incorporated and embodied as part of the present
invention within the scope of the following claims.
EXAMPLE 7
Several samples of ePTFE were impregnated with X1-4105 and Q14010 silicone
diluted in toluene. The ratio of silicone to toluene was 3:1. These
smaples were cured and then submerged in pure toluene. The samples were
removed and inspected for any loss visible degradation in physical
characteristics over the next 72 hours. Although the samples did show
signs of swelling and weight gain, they continued to have good tensile,
abrasion and resilient properties. After removing the samples and allowing
them to dry they were weighted again. The sample with X1-4105 showed a
2.3% weight loss while the sample with Q1-4010 exhibited a 1.3% weight
loss. Since the silicone represetns more than 50% of the compoiste weight
htese losses in weight are insignificant.
When 0.005" thick sheets of silicone rubber were soaked for 72 hours they
swelled, became disfigured and became very sensitive to superficial
damage. The slightest nick or damage to the surface would rapidly
propagate through the sheet when placed under tension.
As such, the composite of the present invention will withstand exposure to
harsh chemicals without significant degradation of the material. The term
"harsh chemicals" is intended to encompass solvents, such as toluene and
similar solutions, as well as other liquid and gaseous materials that are
commonly employed in the cleaning and/or sterilization of cables are wire
jackets. Many of these chemicals will rapidly degrade silicones when not
protected by the ePTFE strucutre of the present invention.
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